874 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



Laramie Group I Ijavc had reference almost entirely to the invertebrate 

 fauna, which consists, so far as the discussions are concerned, entirely 

 of the Mollusca. This was not because the investigation of those sub- 

 jects is more in the line of my special studies, but because being inhabit- 

 ants of the waters in which the formations were deposited, they had a 

 more direct bearing than any others upon the physical phases of the 

 western portion of North America during the period that has been dis- 

 cussed, and, also, because neither the then existing vegetation nor the 

 most important part of the vertebrate fauna was necessarily affected 

 by at least those physical changes which caused an entire change of the 

 whole molluscan ftiuna, both at the beginning and close of the Laramie 

 X)eriod. The reptilian fauna of the Laramie period, however, assumes 

 especial interest, because certain of its types, which extend throughout 

 the whole vertical range of the group, are regarded as characteristic of 

 Cretaceous age. 



Notwithstanding the positive opinions that have been expressed by 

 others upon the subject of the geological age of the Laramie Group, I 

 regard it as still an open question. All paleontologists agree that the 

 Cretaceous period extended at least to the close of the Fox Hills epoch ; 

 and the question is whether the Cretaceous period closed with the close 

 of the Fox Hills epoch or with that of the Laramie period. The question 

 might be extended so as to embrace the inquiry whether the true chrono- 

 logical division between the Cretaceous and Tertiary did not really occur 

 within the Laramie period ; but this, while not unreasonable, would per- 

 haps be inconvenient and unprofitable. That, according to European 

 standards, the Dinosauria which are found even in the uppermost strata 

 of the Laramie Group are of Cretaceous types is doubtless indisputable, 

 and there also appears to be no occasion to question the reference that 

 has been made of fossil plants which have been obtained from even the 

 lowest Laramie strata, to Tertiary types. The invertebrate fossils, of 

 the Laramie Group itself, as I have shown in other writings, are silent 

 as to its geological age, because the types are either unique, are known 

 to exist in both Mesozoic and Tertiary strata, or pertain to living as well 

 as fossil forms.* Every species found in the Laramie Group is no dcubt 

 extinct, but the molluscan types have collectively an aspect so modern 

 that one almost instinctively regards them as Tertiary; and yet some of 

 these types are now known to have existed in the Cretaceous, and even 

 in the Jurassic period. In view of these facts, together with those pre- 

 sented in the foregoing discussions, the following suggestions concern- 

 ing the geological age of the Laramie Group are offered. 



It is a well-known fact that we have in North America no strata 

 which are, according to European standards, equivalent with any part 



* It is a fact worthy of consideration in this connection that a large proportion of the 

 molluscan types of the extensive fresh-water deposits of Southeastern Europe are 

 practically identical with some of those of the Laramie Group, and that Eurox^ean 

 geologists regard those deposits as of Eocene Tertiary age. 



